The invention relates to the introduction of melamine off-gases into a urea plant by means of ejectors.
The off-gases obtained in the melamine synthesis and comprising in particular NH3 and CO2 are usually used for the preparation of urea. Advantageously, the off-gases from the melamine plant are transferred directly into the urea plant, where, for example, they are absorbed in a carbamate stream and transported further into the reactor. An improved, more efficient and more economical method of off-gas introduction is described in K. Abe et al., Kagaku Kogaku 40, 298-302 (1976), in which the off-gases, optionally after removal of residual melamine in a urea scrubber, are introduced directly and unchanged, in the dry state, into the high-pressure zone of the urea plant. This process variant is also described in SU 899538 or WO 98/08808. According to WO 98/32731, the melamine off-gases are first condensed at the pressure of the melamine reactor, the ammonium carbamate formed then being transported into the high-pressure zone of the urea plant.
The disadvantage of the known methods is in particular that in some cases energy losses occur through pressure relief or cooling of the off-gases, with the resulting additional process steps and apparatus components, or that, in the case of direct transfer of the off-gases into the urea plant, the pressure of the off-gases or the pressure in the melamine reactor has to be greater than the pressure in the urea reactor. This necessitates an inflexible and rigidly specified procedure in the two reactors, with the result that the reactors often cannot be operated under the conditions optimal for the respective process.
It has now been found, unexpectedly, that these disadvantages can be eliminated when the melamine off-gases are introduced directly into the high-pressure zone of the urea plant by means of ejectors.
The invention accordingly relates to a method for producing urea, in which the gases (melamine off-gases) which originate from a melamine plant and substantially comprise NH3 and CO2 are introduced directly into the high-pressure zone of a urea plant by means of one or more ejectors. The high-pressure zone of the urea plant includes in particular the reactor, the stripper and the carbamate condenser, as well as lines and apparatus parts present in this region of the plant.
The present method is suitable for any desired melamine plants and urea plants. Such plants are known, for example, from xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistry, 5th ed., vol. A16 (1990), pages 171-185 (Melamine) and vol. A27 (1996), pages 333-365 (Urea), and from K. Abe et al., Kagaku Kogaku 40 (1976), pages 298-302, and from EP-727,414 A, WO 98/08808 and WO 98/32731.
The melamine off-gases used in the present method preferably originate from a melamine high-pressure plant in which melamine is obtained at temperatures of about 300-500xc2x0 C. and pressures of about 80-800 bar from urea with elimination of NH3 and CO2. The off-gases eliminated and substantially comprising NH3 and CO2 are passed, according to the invention, directly into a urea plant, preferably after being passed through molten urea (urea scrubber) for removing residual melamine, as described, for example, in Ullmann. Particularly suitable urea plants are those which are based on the xe2x80x9curea-stripping processxe2x80x9d, the gases being converted into urea at about 150 to 350xc2x0 C., preferably at about 170 to 200xc2x0 C., and about 125 to 350 bar, preferably at about 140 to 200 bar. NH3 and CO2 not converted into urea is expelled in a downstream stripper and then condensed in a condenser, NH3- and CO2-containing ammonium carbamate being formed, which is recycled into the urea reactor. The urea solution emerging from the stripper is concentrated in further downstream decomposers at decreasing pressures, for example in a medium-pressure decomposer, then in a low-pressure decomposer and in a vacuum evaporator, by further decomposition of the carbamates and carbonates present and expulsion of NH3 and CO2. The gases obtained during the concentration are condensed and then recycled into the urea process.
In a preferred method according to the present invention, the ejectors for transporting the melamine off-gases into the high-pressure zone of the urea plant are operated with one or more of the following streams as driving media:
a) liquid NH3,
b) gaseous NH3 or CO2,
c) aqueous solutions containing substantially NH3 and CO2.
The aqueous solutions containing substantially NH3 and CO2 are preferably obtained in a urea plant but they may also originate from other processes in which NH3 and CO2 are obtained, for example from a melamine plant. Aqueous solutions which originate in the preparation of urea, in particular from the working-up, contain substantially NH3 and CO2 and originate, for example, from the carbamate condenser or from the low-pressure zone of the urea plant, for example from the medium-pressure absorber, can be used according to the invention, as a driving medium for the ejectors for introducing the melamine off-gases into the high-pressure zone of the urea plant.
A particular advantage of using the solutions originating from the bottom of the medium-pressure absorber and also containing carbonates in addition to NH3, CO2, H2O and carbamates is that they have a low expulsion pressure of the dissolved gases. In fact, it proves advantageous if the gas expulsion pressure of the propellants, optionally also the temperature, are lower than the pressure and optionally the temperature of the melamine off-gases to be introduced. This prevents expulsion of dissolved gases in the ejector.
Depending on the type and amount of the driving medium, the pressure of the driving media used is such that as far as possible the total amount of off-gases obtained can be transported, depending on the suction conditions and the amount of the off-gases to be conveyed and on the respective counter-pressure in the high-pressure zone of the urea plant. The pressure and the amount of the driving media must accordingly be correspondingly high in order to ensure that the off-gases to be transported can be introduced in the respective amount at the respective pressure in the high-pressure zone of the urea plant. The pressure of the driving media is higher than the pressure in the high-pressure zone of the urea plant and is preferably from 1.1 to 3 times as high, particularly preferably from 1.3 to 2.5 times as high, as the pressure in the high-pressure zone of the urea plant. The temperature of the driving medium is in particular dependent on the respective procedure and is preferably in a range from about 10xc2x0 C. to 200xc2x0 C. In the case of the use of NH3, for example of the synthesis NH3 for the urea preparation, a temperature of from about 10xc2x0 C. to 80xc2x0 C., particularly preferably from about 20xc2x0 C. to 65xc2x0 C., proves advantageous. In the case of the use of synthesis CO2, the temperature is preferably slightly higher, from about 115xc2x0 C. to 140xc2x0 C. NH3- and CO2-containing aqueous solutions as driving media which originate, for example, from the carbamate condenser of the urea plant, for example from 3 to 4 bar steam simultaneously being generated, preferably have temperatures of from about 150xc2x0 C. to 160xc2x0 C. Recycled carbonate solutions, for example from the medium-pressure absorber of the urea plant, can have temperatures of from about 65xc2x0 C. to 100xc2x0 C., preferably from about 65xc2x0 C. to 70xc2x0 C.
The molar ratio of NH3 to CO2 in the melamine off-gases to be introduced and originating from the melamine plant depends on the type of melamine process used and is preferably from about 2.5 to 5. The pressure of the melamine off-gases originating from the melamine plant corresponds substantially to the pressure in the melamine reactor and is preferably from about 50 to 250 bar, particularly preferably from about 70 to 200 bar. The temperature of the melamine off-gases is preferably from about 175 to 250xc2x0 C., particularly preferably from about 180 to 210xc2x0 C.
Accordingly, the particular advantage of the process according to the invention is in particular the use of the product streams to be recycled as ejector driving media and also the use of the NH3 and CO2, used as starting materials, as ejector driving media. Consequently, on the one hand, an economical procedure is achieved and, on the other hand, it is also possible to operate the urea and melamine production in a flexible manner and under optimum conditions in each case, melamine reactor pressures lower than urea reactor pressures also being possible.
FIG. 1 to FIG. 4 show, by way of example, 4 possible variants according to the invention for introducing the melamine off-gases into the urea reactor with the aid of ejectors. FIG. 1 to FIG. 4 show the following substantial parts of the urea plant and of the melamine plant:
R-1 Urea reactor
C Carbamate condenser
ST-1 Urea stripper
Z-1 Decomposer/separator
K-1 Condenser 1
K-2 Condenser 2
K O-1 Medium-pressure absorber
E-1 Ejector 1
E-2 Ejector 2
E-3 Ejector 3
P-1 Carbamate pump
P-2 Carbonate pump
P-3 NH3 pump
R-2 Melamine reactor
ST-2 Melamine stripper
W Urea scrubber
VZ Dwell tank
PK Product cooler